It is known that certain organic compounds (“photosensitisers”) can induce cell death by absorption of light in the presence of oxygen. The cytotoxic effect involves Type I and/or Type II photooxidation. Such photosensitisers find use in the treatment of cancer and other diseases or infections with light (photodynamic therapy) and in the sterilisation (including disinfection) of surfaces and fluids by the light-induced destruction of microbes. In this context, the term sterilisation is taken to mean the reduction or elimination of microbes in a particular situation. For prevention of wound infections sterilisation means a significant reduction in bacterial load on, in or around a wound site which helps to promote efficient wound healing or which minimises the chance that wound infection will occur.
It is also known that certain coloured phenothiazinium compounds, (e.g. methylene blue) can take part in Type I and Type II photooxidation processes, but compounds of this type thus far have proved unsuitable or of low efficacy as sensitisers for photodynamic therapy, or have shown low photochemical antimicrobial activity.
For application in PDT, a good sensitiser must have at least some and preferably all of the following properties. Most importantly, it should cause the destruction of target cells (for example tumour cells or bacterial cells) efficiently on exposure to light (preferably wavelengths ca. 600-800 nm). The PDT treatment using the photosensitiser should show a high degree of selectivity between target and normal tissues. The sensitiser should have relatively little dark toxicity and it should cause little or no skin photosensitivity in the patient. The sensitiser should have short drug to light intervals for patient and hospital convenience and to minimise treatment costs.
For applications in photosterilisation, a good sensitiser must show a strong phototoxic effect in a wide range of microorganisms, ideally using ambient light, and should not photobleach readily.
In oncology, several different types of photosensitiser have been used to treat both solid tumours and thin tumours of hollow organs such as the oesophagus and bladder. However, the use of these photosensitisers has been restricted partly because of lack of selectivity between tumour and healthy tissue and partly because of the prolonged skin photosensitivity which can be caused. There is a need for new photosensitisers which cause little or no skin photosensitivity and which selectively destroy tumour cells.
Although PDT has previously been used in the treatment of tumours, it has not yet been used clinically against infections caused by bacteria and other microorganisms. The use of antibiotics to treat bacterial infections is becoming challenging due to the increasing resistance of many bacterial species to commonly used antibiotics, such as tetracyclines and beta-lactams. Hospital-acquired antibiotic resistant infections such as MRSA are especially problematic. Photodynamic antibacterial treatment is a promising alternative to antibiotics for local treatment.
When developing antibacterial agents a major problem which must be overcome is the uptake of the drug into the bacterial cell. Gram negative and Gram positive bacteria differ in the composition of their outer surface and respond differently to antimicrobial agents, especially in terms of uptake. Due to the high negatively charged surface of Gram negative bacteria they are relatively impermeable to neutral or anionic drugs, including most commonly used photosensitisers. Development of antimicrobial photosensitisers which are effective against Gram negative bacteria, as well as Gram positive bacteria would be highly beneficial to replace commonly used antibiotics and drugs which are becoming increasingly ineffective due to resistance.
A number of different types of photosensitiser have been investigated in bacteria. These include phenothiazinium compounds, phthalocyanines, chlorins and naturally occurring photosensitisers. For uptake into Gram negative bacteria, it is accepted that the cationic derivatives are the most effective.
Phenothiazinium compounds are blue dyes with maximum absorption at wavelengths between 600-700 nm. They have been studied for their non-photodynamic antibacterial properties but few apart from methylene blue and toluidine blue have been investigated photodynamically.
Wainwright et al (1998) investigated the effect of a series of phenothiazinium methylene blue derivatives in tumour cell lines and bacteria. New methylene blue (NMB) and di methyl methylene blue (DMMB) were effective at inactivating MRSA and were shown to be more effective photosensitisers than methylene blue when acting on pigmented melanoma cell lines. Wagner et al (1998) studied these dyes and in addition a hydrophobic derivative for the inactivation of enveloped viruses.
The precise mode of antibacterial action of methylene blue is unknown, but one hypothesis is that because of its stereochemistry it can intercalate into DNA, and that photodynamic action causes DNA damage. Methylene blue itself has been shown to be ineffective as an anti-tumour agent. In addition, methylene blue is known to be susceptible to photobleaching, which could be a serious disadvantage in some applications. Because of the recognised limitations of methylene blue, both anti-tumour PDT and antimicrobial PDT would benefit from development of new phenothiazinium-based photosensitisers.